1. Field of the Invention
The present invention relates to a radio receiver that is operable with respect to different transmission rates and, more particularly, to a circuit and method for automatically controlling the gain of a radio received signal.
2. Description of the Related Art
With a recent increase in the number of subscribers to a radio selective calling system, an increase in the transmission rate (hereinafter referred to as "bit rate") has been attempted. By increasing the bit rate, it is possible to increase the amount of data received and thereby prevent a rise In the channel occupation factor. There has been proposed a method of changing the bit rate during transmission of a transmission signal to achieve the increased amount of transmission data.
Also, it has been known that the intermodulation (IM) caused by a high electric field of the adjacent frequency channel has a bad effect upon the receiving characteristics of the current channel in use. In order to suppress such intermodulation interference, a radio selective call receiver is generally provided with an automatic gain control (AGC) circuit for use in countermeasure against the intermodulation. More specifically, such an AGC circuit controls a high-frequency amplifier such that its gain decreases as the input level of the demodulator is increased.
Meanwhile, it has been known that the input level of the demodulator becomes higher in proportion to the increase in the bit rate. Therefore, when constructing the AGC circuit for use in countermeasure against the intermodulation, it is also necessary to take the problems resulting from such phenomenon into consideration.
For example, in Japanese Patent Application Laid-Open Publication No. 4-150232, there has been proposed a demodulator that is provided with a plurality of attenuators, one of which is selected depending on the bit rate. Such a conventional demodulator can solve the problem that the dynamic range of the AGC circuit enlarges with an increase in the bit rate.
However, there are developed drawbacks in the conventional AGC circuit which controls a high-frequency amplifier such that its gain decreases as the input level of the demodulator is increased. Since the operating point is fixed to a single point regardless of the bit rate. there were the problems that when the bit rate was changed, the sensitivity was unnecessarily decreased, the necessary gain control became inoperative, etc.
For example, when a radio selective calling signal has been changed from a low bit rate to a high bit rate, the input level of the demodulator increases. As a result, the AGC circuit operates with the result that the gain of the high frequency amplifier decreases, which results in a degradation of the receiving sensitivity at the high bit-rate time. To prevent this, when setting the operating point of the AGC circuit in conformity with the received signal level at the high bit-rate time. the AGC circuit does not operate this time at the low bit-rate time unless the input level is high.
Also, in the construction provided with an attenuator for each of a plurality of bit rates, disclosed in Japanese Patent Application Laid-Open No. 4-150232, it is difficult to promote the miniaturization and simplification of the circuit.